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Question Number 89878 by malwaan last updated on 19/Apr/20
A four_digit whole number  is interesting if the number  formed by the leftmost two  digits is twice as large as the  number formed by the  rightmost two digits.  (for example 2010 is interesting)  1  find the largest whole number  B such that all interesting  numbers are divisible by B  2  find the smallest whole  number D such that D is  divisible by all interesting  numbers.
$$\boldsymbol{{A}}\:\boldsymbol{{four\_digit}}\:\boldsymbol{{whole}}\:\boldsymbol{{number}} \\ $$$$\boldsymbol{{is}}\:\boldsymbol{{interesting}}\:\boldsymbol{{if}}\:\boldsymbol{{the}}\:\boldsymbol{{number}} \\ $$$$\boldsymbol{{formed}}\:\boldsymbol{{by}}\:\boldsymbol{{the}}\:\boldsymbol{{leftmost}}\:\boldsymbol{{two}} \\ $$$$\boldsymbol{{digits}}\:\boldsymbol{{is}}\:\boldsymbol{{twice}}\:\boldsymbol{{as}}\:\boldsymbol{{large}}\:\boldsymbol{{as}}\:\boldsymbol{{the}} \\ $$$$\boldsymbol{{number}}\:\boldsymbol{{formed}}\:\boldsymbol{{by}}\:\boldsymbol{{the}} \\ $$$$\boldsymbol{{rightmost}}\:\boldsymbol{{two}}\:\boldsymbol{{digits}}. \\ $$$$\left(\boldsymbol{{for}}\:\boldsymbol{{example}}\:\mathrm{2010}\:\boldsymbol{{is}}\:\boldsymbol{{interesting}}\right) \\ $$$$\mathrm{1}\:\:\boldsymbol{{find}}\:\boldsymbol{{the}}\:\boldsymbol{{largest}}\:\boldsymbol{{whole}}\:\boldsymbol{{number}} \\ $$$$\mathbb{B}\:\boldsymbol{{such}}\:\boldsymbol{{that}}\:\boldsymbol{{all}}\:\boldsymbol{{interesting}} \\ $$$$\boldsymbol{{numbers}}\:\boldsymbol{{are}}\:\boldsymbol{{divisible}}\:\boldsymbol{{by}}\:\mathbb{B} \\ $$$$\mathrm{2}\:\:\boldsymbol{{find}}\:\boldsymbol{{the}}\:\boldsymbol{{smallest}}\:\boldsymbol{{whole}} \\ $$$$\boldsymbol{{number}}\:\mathbb{D}\:\boldsymbol{{such}}\:\boldsymbol{{that}}\:\mathbb{D}\:\boldsymbol{{is}} \\ $$$$\boldsymbol{{divisible}}\:\boldsymbol{{by}}\:\boldsymbol{{all}}\:\boldsymbol{{interesting}} \\ $$$$\boldsymbol{{numbers}}. \\ $$
Commented by malwaan last updated on 19/Apr/20
1  B = 201   why ?
$$\mathrm{1}\:\:\mathbb{B}\:=\:\mathrm{201}\:\:\:{why}\:? \\ $$
Answered by malwaan last updated on 20/Apr/20
0201=201×1  0402=201×2  .  .  2010=201×10  2211=201×11  .  .  9648=201×48  9849=201×49  ⇒201 is GCD  ∴ B = 201
$$\mathrm{0201}=\mathrm{201}×\mathrm{1} \\ $$$$\mathrm{0402}=\mathrm{201}×\mathrm{2} \\ $$$$. \\ $$$$. \\ $$$$\mathrm{2010}=\mathrm{201}×\mathrm{10} \\ $$$$\mathrm{2211}=\mathrm{201}×\mathrm{11} \\ $$$$. \\ $$$$. \\ $$$$\mathrm{9648}=\mathrm{201}×\mathrm{48} \\ $$$$\mathrm{9849}=\mathrm{201}×\mathrm{49} \\ $$$$\Rightarrow\mathrm{201}\:\boldsymbol{{is}}\:\boldsymbol{{GCD}} \\ $$$$\therefore\:\mathbb{B}\:=\:\mathrm{201} \\ $$

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